Detecting the presence of railroad vehicles on a trackway is a problem which was originally solved with the invention of the track circuit. A track circuit is merely an electrical circuit in which electrical energy is applied to a section of railroad track at one point, and a detector of electrical energy is applied to another point of the railroad track. When a train enters the trackway between the transmitter and the receiver (typically the entrance end of a track section is in the vicinity of the receiver's connection), the steel wheel-axle combination shunts electrical energy away from the detector, and this lack of energy at the detector is used to indicate the presence of the train. In early track circuit applications, the circuit was well-defined by insulating the track rails at the boundaries of the track circuit. Thus, each track circuit could, for example, include only a single source of energy and accordingly, the need for elaborate measures to prevent false energization of the energy detector were minimal.
This state of affairs has changed radically with the use of rails in which track circuit insulated joints are eliminated. Elimination of the insulated joints required further measures to prevent spurious electrical energy from reaching the detector which energy has the potential for masking the presence of the train. One technique which has been adopted is the use of different frequencies of track frequency energy in adjacent track circuits, and "tuning" of the receiver to the appropriate frequency.
In an effort to expand the utility of track circuits, additional signalling currents have been imparted into them for the purpose of transmitting speed control information to the train. In these circuits, train information signals are carried by signalling current of a frequency which is different from the track frequency of any of the track circuits. One popular technique for transferring information to a train via a track circuit is to use one or just a few train information frequency carriers, but to modulate those carriers at different rates depending upon the speed control information sought to be transmitted.
In a majority of track circuits which have the capability of transmitting train information, the train information is carried by track currents flowing in the same rails which carry the train detection signalling currents. As a result, typical track circuits in use today are composed of transmitting and receiving equipment. The receiving equipment is used to detect the presence of track frequency currents and, when such currents are detected, to energize a relay to indicate the unoccupancy of the associated track section, and the transmitting equipment is used to generate both the track frequency and the train information signalling currents for application to the track circuit. In addition, for added security, track frequency carrier is modulated at one of a number of code rates.
An analysis of the operation of a track circuit will illustrate that the track relay must be capable of being energized by the track frequency signal in the presence of the train information signal, for this signal combination exists in every track circuit as the train occupying the track circuit exits the track circuit.
Since there was a desire to use modulated track frequency signal currents, and since there is the necessity of modulating the train information signalling currents, and since they typically flow in the track rails, the practice has grown up of using the train information code rate to modulate the track frequency energy. Partly, this is a result of the necessity for the track relay to pick up in the presence of train information modulated carrier. Thus, in effect, the transmitter power amplifier and track circuit is time shared at a code rate of the train information signal. When the train information carrier signal is on, the track frequency carrier is off and vice versa. This necessarily means that the track frequency carrier may be modulated at any of the code rates used for transmitting train information. And accordingly, the element in the train detection processing chain which detects the track frequency modulation must be of a characteristic which will accept any modulation rate within this range. For example, in a typical application, the modulation rates used are from 1.25 Hz. to 21.5 Hz. While these arrangements have operated quite well, and are actually in use in a number of rapid transit systems today, we have discovered that certain improvements are necessary and desirable.
Contemporaneous with the development of track circuits, briefly outlined above, the control arrangements for train power equipment has also been changing such that today modern control arrangements include pulse type control devices (for example, silicon control rectifiers or equivalent). The use of these pulse type devices along with the relatively larger amounts of power they switch (as compared to the signal circuits) can result in spectrally rich currents induced in the wayside equipment including the wayside track receiver. This has required the noise immunity of the track receiver to be as high as possible. However, the relatively broadly tuned modulation detection element hinders increasing the noise immunity of the track receiver.
Therefore, it is one object of the present invention to increase the noise immunity of the track circuit receiver. It is another object of the present invention to improve the noise immunity of the track receiver by sharply tuning the modulation detection element. It is another object of the invention to modulate track frequency carrier energy at a fixed rate so as to allow sharply tuning the modulation rate to which the receiver responds.